This is really quite easy once you know how to do it. The problem is, there are a lot of moving pieces so it can be very difficult the first time you try. You'll want to read up on HTTP and XMLHttpRequest as lostjimmy recommends and DHTML. Though do not try to do the requests yourself. They are painful, difficult, and verbose to do in a x-platform way. Use a JS lib. I personally recommend jQuery. I've worked with a few of them and it's the most facile and Perl-like to me.

Before you proceed however, make sure that the feature isn't a security risk, which it sounds like to me. If, for example, I could put javahead into a field and automatically see personal information on PM, it would be a problem. :) If this is for an app like an HR frontend or something which only authorized individuals will be using, then it would be a nice feature. Otherwise, it's probably something to avoid.

(update: I wrote a little sample script doing a simplistic version of this stuff. I'll post it later today when I have time to add comments and info.)